Release Planning is the process of choosing which features, or stories, will be included in a given release. Usually, either the feature set or the release date is fixed, and the other is variable. Either way, this is a business decision. On a large software project, it would be imprudent not to build in a time “buffer” for contingencies (Swine Flu, anyone?). You do this at the release planning level. You can buffer for time or features, or both.
Tools that are useful in release planning include user personas, Kano analysis, paper prototyping and story mapping.